Nearly all cancers are treatable in their earliest forms, but often fatal once metastasis has occurred. Improved diagnosis and therapy of cancer will result from a more directed approach in which antigens specific to or over expressed on tumors are targeted. Although therapies targeted to a single antigen have proven moderately successful, poor specificity and affinity, coupled with resistance have limited their use. The goal of this research is to develop a bacteriophage (phage)-based platform for targeting multiple cancer antigens regardless of spatial or biological relatedness. The advantage to dual targeting includes both increased avidity through decreased simultaneous dissociation rates, and increased selectivity through multiple antigen binding. Numerous receptors have been shown to play key roles in tumorigenesis but have proven difficult to target. For example, the ErbB2 and ErbB3 receptors are both present in early stage and resistant carcinomas, but absent or present at low levels in normal tissues. Although the receptors can heterodimerize, the majority do not and are not close enough for targeting by current heterovalent platforms, such as bispecific antibody constructs. Phage provide a scaffold that spans hundreds of nanometers and display multiple copies of targeting peptides, which can be utilized to bind antigens regardless of interaction or separation. It is hypothesized that phage displaying targeted peptides at opposite tips of the virion can simultaneously bind multiple cancer antigens (such as Erb2 and Erb3) regardless of their proximity or interaction status. Such heteromultivalent (HMV) phage, engineered to appropriate lengths, will be employed in a novel, amplified 3-step pretargeting approach. Truncated, biotinylated HMV phage will be injected for in vivo tumor targeting. Following phage clearance, fluorescent streptavidin-coated silica nanoparticles will be used to target the multiple biotins on tumor-bound phage. The nanoparticles have demonstrated rapid blood clearance and will allow for multimodal imaging and a second level of signal amplification. Biotin conjugated to the 1,4,7,10- tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-chelated radionuclide [177Lu] lutetium will deliver high levels of a theranostic agent that can be simultaneously used for single photon emission computed tomography (SPECT) imaging in addition to providing a high dose of tumor-localized beta radiation. The specific aims include: 1) genetically engineer heteromultivalent (HMV) nanophage of discrete lengths (50-750 nm) displaying ErbB2 and ErbB3 targeting peptides, 2) characterize HMV nanophage affinity, specificity, and avidity to ErbB2/ErbB3, and 3) characterize the tumor targeting and pharmacokinetics of the nanophage using the amplified three-step pretargeting approach in ErbB2+/ErbB3+ BT-474 human breast cancer xenografted mice. These phage will yield a new imaging and therapeutic platform with high affinity, selectivity and optimum pharmacokinetics. Furthermore, the phage can be engineered to display targeting peptides or antibodies for any antigen(s) of choice, providing great versatility as a novel theranostic agent.